Hayman Robin M, Jeffery Kathryn J
Institute of Behavioural Neuroscience, Department of Cognitive, Perceptual and Brain Sciences, Division of Psychology and Language Sciences, University College London, London WC1H 0AP, UK.
Hippocampus. 2008;18(12):1301-13. doi: 10.1002/hipo.20513.
How entorhinal grids generate hippocampal place fields remains unknown. The simplest hypothesis-that grids of different scales are added together-cannot explain a number of place field phenomena, such as (1) Summed grids form a repeating, dispersed activation pattern whereas place fields are focal and nonrepeating; (2) Grid cells are active in all environments but place cells only in some, and (3) Partial environmental changes cause either heterogeneous ("partial") remapping in place cells whereas they result in all-or-nothing "realignment" remapping in grid cells. We propose that this dissociation between grid cell and place cell behavior arises in the entorhinal-dentate projection. By our view, the grid-cell/place-cell projection is modulated by context, both organizationally and activationally. Organizationally, we propose that when the animal first enters a new environment, the relatively homogeneous input from the grid cells becomes spatially clustered by Hebbian processes in the dendritic tree so that inputs active in the same context and having overlapping fields come to terminate on the same sub-branches of the tree. Activationally, when the animal re-enters the now-familiar environment, active contextual inputs select (by virtue of their clustered terminations) which parts of the dendritic tree, and therefore which grid cells, drive the granule cell. Assuming this pattern of projections, our model successfully produces focal hippocampal place fields that remap appropriately to contextual changes.
内嗅皮层网格如何产生海马体位置场仍然未知。最简单的假设——不同尺度的网格相加——无法解释一些位置场现象,比如:(1)相加后的网格形成重复、分散的激活模式,而位置场是集中且不重复的;(2)网格细胞在所有环境中都活跃,而位置细胞只在某些环境中活跃;(3)局部环境变化会导致位置细胞出现异质性(“部分”)重映射,而在网格细胞中则会导致全或无的“重新排列”重映射。我们提出,网格细胞和位置细胞行为之间的这种分离出现在内嗅皮层-齿状回投射中。我们认为,网格细胞/位置细胞投射在组织和激活方面都受环境调节。在组织方面,我们提出,当动物首次进入新环境时,来自网格细胞的相对均匀的输入会通过树突状树中的赫布过程在空间上聚集,这样在相同环境中活跃且具有重叠场的输入就会在树的同一子分支上终止。在激活方面,当动物重新进入现在熟悉的环境时,活跃的环境输入会(凭借它们聚集的终止点)选择树突状树的哪些部分,进而选择哪些网格细胞来驱动颗粒细胞。假设这种投射模式,我们的模型成功产生了集中的海马体位置场,这些位置场会根据环境变化进行适当的重映射。